Aluminum alloy sheet

ABSTRACT

A 7xxx-series aluminum alloy sheet produced by a common procedure is allowed to include a surface part having a texture with grown Cube orientation, and a central part having a texture with grown S orientation. Namely, the surface part and the thickness-central part in the sheet are allowed to have different textures that are optimal respectively for shock absorption and for strength. This allows the aluminum alloy sheet to have better shock absorption upon automobile collision without lowering its strength, where the shock absorption is evaluated by a VDA bend test as illustrated in FIG.  1.

TECHNICAL FIELD

The present invention relates to a 7xxx-series aluminum alloy sheet which is produced by common rolling, which has high strength, and which still offers excellent shock absorption.

BACKGROUND ART

When taking automobiles as an example of structural materials, social demands for weight reduction of automobile bodies have become higher and higher in consideration typically of global environment. To meet these demands, aluminum alloy materials are applied, instead of part of ferrous materials such as steel sheets, to panels (such as hoods, doors, and roofs, and other outer panels, and inner panels) and reinforcements such as bumper reinforcements (bumper R/F) and door beams, of the automobile bodies.

However, further weight reduction of automobile bodies requires the application of aluminum alloy materials more widely even to frames, pillars, and other structural components, which particularly contribute to weight reduction among automobile components. These automobile structural components require higher strength of materials as compared with the automobile panels, and should be formed using Japanese Industrial Standards (JIS) or The Aluminum Association Standards (AA) 7xxx-series aluminum alloys, which have been already used as the reinforcements.

The automobile structural component reinforcements such as bumper reinforcements and door beams have already been generally used and are obtained from extruded articles produced by hot-extruding 7xxx-series aluminum alloys. In contrast, frames, pillars, and other large-sized structural components are preferably obtained from material rolled sheets which are produced by a common procedure typically including soaking of ingots, subsequently hot-rolling the soaked ingots, or further cold-rolling the hot-rolled articles. However, rolled sheets of 7xxx-series aluminum alloys have been used in practice little, because these alloys are high alloys and are difficult to be formed into rolled sheets.

However, investigations to control the texture of such a high-strength 7xxx-series aluminum alloy rolled sheet have been made so as to apply it to not only the automobile structural components, but also structural components typically of railroad vehicles. The high-strength 7xxx-series aluminum alloy rolled sheet is hereinafter also referred to as a “7xxx-series aluminum alloy sheet” or simply referred to as a “sheet”.

For example, Patent Literature (PTL) 1 and PTL 2 disclose techniques of controlling the texture of a 7xxx-series aluminum alloy sheet. According to the techniques, ingots are forged and then subjected to rolling in the warm working region repeatedly to allow the sheet to have a finer or microstructure. The techniques are intended to provide higher strength and higher resistance to stress corrosion cracking (SCC) of the sheet for structural components. The refinement of the sheet microstructure is performed so as to give a texture including 25% or more of small angle grain boundaries having a misorientation of 3° to 10° while controlling or restraining large angle grain boundaries having a misorientation of 20° or more, because the large angle grain boundaries cause a potential difference between the grain boundary and the inside of the grain, which potential difference in turn causes deterioration in SCC resistance.

The techniques disclosed in PTL 1 and PTL 2 employ the special repeated warm rolling procedures, because common production methods using hot rolling (optionally in combination with cold rolling) fail to give such a texture including small angle grain boundaries in a large amount. The production methods according to the techniques significantly differ in process firm common production methods and are not practical as methods for producing the target sheet.

In contrast, PTL 3 discloses a technique of controlling the texture of a 7xxx-series aluminum alloy sheet to provide a sheet which is produced by a common sheet production method, which has strength and SCC resistance both at excellent levels, and which is for use in automobile members.

Specifically, this sheet is an Al—Zn—Mg aluminum alloy sheet including Zn in a content of 3.0% to 8.0% and Mg in a content of 0.5% to 4.0% with the remainder consisting of Al and unavoidable impurities. The sheet includes a texture having an average grain size of 15 μm or less and having an average total area percentage of grains with Brass orientation, S orientation, or Cu orientation of 30% or more.

Specifically, the aluminum alloy sheet does not include a common equiaxial recrystallized microstructure, but rather includes a fibrous microstructure which is analogous to the microstructure in the extruded article, where the fibrous microstructure is specified as having the Brass orientation, S orientation, and/or Cu orientation.

PTL 3 mentions that the sheet, as having the texture as above, can have such a microstructure as to allow strain, if applied on the sheet, not to locally concentrate, but uniformly dislocate. This literature also mentions that the technique allows even a 7xxx-series aluminum alloy sheet produced by a common procedure to have a high strength in terms of 0.2% yield strength of 350 MPa or more, to have a large elongation to surely offer satisfactory formability, and to resist deterioration in SCC resistance in spite of having a high strength.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2001-335874

PTL 2: JP-A No. 2002-241882

PTL 3: JP-A No. 2014-62285

SUMMARY OF INVENTION Technical Problem

With increasing levels of recent automobile collision safety standards, the frames, pillars, and other automobile structural components are required, typically in Europe, to have satisfactory shock absorption (crushing properties) upon automobile collision, where the shock absorption is evaluated according to “VDA 238-100 Plate bending test for metallic materials standardized by the German Automotive Industry Association (Verband der Automobilindustrie; VDA)”. This test is hereinafter referred to as a “VDA bend test”.

However, neither sheets as described in PTL 1 and PTL 2 nor sheets as described in PTL 3 can meet the shock absorption (crushing properties) upon automobile collision according to such rigorous safety standards. The former sheets have a microstructure which is a texture including a large proportion of small angle grain boundaries at a misorientation of 3° to 10° while including a smaller proportion of large angle grain boundaries. The latter sheets have a texture including a fibrous microstructure.

There has not yet been found, and there is a room for elucidation of, effective techniques or means to allow a 7xxx-series aluminum alloy sheet produced by common rolling to meet the shock absorption (crushing properties) upon automobile collision without lowering strength of the sheet.

Under these circumstances, the present invention has an object to provide a 7xxx-series aluminum alloy sheet which is produced by common rolling and which still offers better shock absorption (crushing properties) upon automobile collision without lowering its strength.

Solution to Problem

To achieve the object, the present invention provides an aluminum alloy sheet containing, in mass percent, Zn in a content of 2.0% to 9.0% and Mg in a content of 0.5% to 4.5%, and being controlled to have a Cu content of 0.5% or less (including 0%), a Zr content of 0.15% or less (including 0%), a Mn content of 0.2% or less (including 0%), a Cr content of 015% or less (including 0%), and a Sc content of 0.05% or less (including 0%), with the remainder consisting of Al and unavoidable impurities. The aluminum alloy sheet includes a surface part ranging frtim a sheet surface to a depth of 15% of a sheet thickness, and a thickness-central part. Of all grains in the suiface part, grains with Cube orientation are present in an area percentage [surface part Cube], and grains with S orientation are present in an area percentage [surface part S]. Of all grains in the thickness-central part, grains with Cube orientation are present in an area percentage [thickness-central part Cube], and grains with S orientation are present in an area percentage [thickness-central part S]. The surface part includes an equiaxial recrystallized mianstructure having an average grain size of 40 μm or less. The surface part and the thickness-central part have different textures as follows. The [surface part Cube] is 10% or more. The [surface part S] is 10% to 40%. The ratio of the [surface part Cube] to the [thickness-central part Cube] is greater than 1.0. The ratio of the [surface part S] to the [thickness-central part S] is less than 1.0.

Advantageous Effects of Invention

The inventors of the present invention focused attention on the texture of a 7xxx-series aluminum alloy sheet after solution treatment and quenching and analyzed the relationship between the texture and the shock absorption (crushing properties) upon automobile collision, which shock absorption is evaluated by a VDA bend test. As a result, the inventors have found that the shock absorption is affected particularly by the texture of a sheet surface part (surface layer) and can be improved by allowing the sheet surface part to have a specific texture with developed Cube orientation. The inventors have also found that this sheet can maintain its strength by allowing a sheet central part (thickness-central part) to have a specific texture with developed S orientation.

Specifically, the inventors have found that control of the surface part and the thickness-central part of the sheet to have different textures optimal respectively to shock absorption and to strength allows the sheet to have better shock absorption upon automobile collision without lowering its strength.

As described above, the present invention allows a 7xxx-series aluminum alloy sheet produced by common rolling to have strength and shock absorption both at satisfactory levels by allowing the surface part and the thickness-central part to have different textures. Accordingly, the present invention can provide a 7xxx-series aluminum alloy sheet that is advantageously suitable for structural components typically of the automobiles and railroad vehicles, which require these properties.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a perspective view and illustrates how to perform a VDA bend test to evaluate shock absorption.

DESCRIPTION OF EMBODIMENTS

As used herein, the term “aluminum alloy sheet” refers to a 7xxx-series aluminum alloy sheet produced by a common rolling process, in which ingots are soaked, hot-rolled, and further cold-rolled to give a cold-rolled sheet, and the cold-rolled sheet is further subjected to a heat treatment or treatments such as solution treatment. In other words, the term “aluminum alloy sheet” does not include sheets produced by a special rolling process or production method such as the rolling process as disclosed in PTL 1 and PTL 2, in which ingots are forged and subjected to warm rolling repeatedly in many times.

Such 7xxx-series aluminum alloy sheets as above are subjected to press forming and/or working including stretch flanging (burring and bore expanding) and formed into structural components typically of automobiles, bicycles, and railroad vehicles.

Embodiments of the present invention will be specifically described below on condition to condition basis.

Aluminum Alloy Chemical Composition

Initially, the chemical composition of the aluminum alloy sheet according to the present invention will be described below, including reasons for specifying the contents of elements. All percentages for contents of elements are by mass.

The chemical composition of the aluminum alloy sheet according to the present invention is determined as an Al—Zn—Mg 7xxx-series aluminum alloy so as to surely have properties required of structural components typically of automobiles, such as strength, shock absorption (crushing properties), and SCC resistance. From this viewpoint, the aluminum alloy sheet according to the present invention contains, as its chemical composition in mass percent, Zn in a content of 2.0% to 9.0% and Mg in a content of 0.5% to 4.5% and is controlled to have a Cu content of 0.5% or less (including 0%), a Zr content of 0.15% or less (including 0%), a Mn content of 0.2% or less (including 0%), a Cr content of 0.15% or less (including 0%), and a Sc content of 0.05% or less (including 0%), with the remainder consisting ofAl and unavoidable impurities.

The chemical composition may further include either one or both of Ag in a content of 0.01% to 0.2% and Sn in a content of 0.001% to 0.1%, in mass percent. In addition to, or instead of this, the chemical composition may further include Ti in a content of 0.001% to 0.1% in mass percent.

Zn: 2.0% to 9.0%

Zinc (Zn), which is an essential alloy element, forms clusters (fine precipitates) with magnesium (Mg) during natural aging at room temperature after solution treatment, and contributes to better work hardening properties. This element also forms aged precipitates during artificial aging, and contributes to higher strength. The aluminum alloy sheet, if having a Zn content of less than 2.0 mass percent, has insufficient strength, fails to control the textures within the specified ranges, and may thereby have strength and formability in inferior balance. In contrast, the aluminum alloy sheet, if having a Zn content of greater than 9.0 mass percent, causes a larger amount of grain boundary precipitate MgZn₂, becomes susceptible to grain-boundary corrosion, and has inferior corrosion resistance. To eliminate or minimise these, of the Zn content, the lower limit is 2.0%, and preferably 3.7%; and the upper limit is 9.0%, and preferably 8.3%.

Mg: 0.5% to 4.5%

Magnesium (Mg), which is an essential alloy element, forms clusters (fine precipitates) with Zn during natural aging at room temperature after solution treatment, and contributes to better work hardening properties. This element also forms aged precipitates during artificial aging, and contributes to higher strength. The aluminum alloy sheet, if having a Mg content of less than 0.5%, has insufficient strength; and, if having a Mg content of greater than 4.5 mass percent, suffers from casting cracks and has lower rolling properties. This may impede production and trial production of the sheet. To eliminate or minimize these, of the Mg content, the lower limit is 0.5%, and preferably 1.4%; and the upper limit is 4.5%, and preferably 4.3%. To provide necessary strength, the contents of Mg and Zn are preferably controlled in good balance.

Cu, Zr, Mn, Cr, and Sc

Copper (Cu), zirconium (Zr), manganese (Mn), chromium (Cr), and scandium (Sc) each cause the sheet to have a significantly higher recrystallization temperature upon solution treatment and impede the development of Cube orientation in the surface part due to a high heating temperature for inducing recrystallization of the sheet in the solution treatment, where the surface part is a portion ranging from the sheet surface to a depth of 15% of the sheet thickness. Accordingly, the aluminum alloy sheet, if containing these elements in excessively high contents, may fail to have a high [surface part Cube] as specified in the present invention, where the surface part Cube] is the area percentage of grains with Cube orientation in the surface part. In addition, the aluminum alloy sheet, if containing these elements in excessively high contents, may fail to have a [surface part S] controlled within the range specified in the present invention, where the [surface part S] is the area percentage of grains with S orientation. This is because these elements promote the development of grains with S orientation in the surface part.

On the basis of these, the aluminum alloy sheet according to the present invention should be positively controlled to have a Cu content of 0.5% or less (including 0%), a Zr content of 0.15% or less (including 0%), a Mn content of 0.2% or less (including 0%), a Cr content of 0.15% or less (including 0%), and a Sc content of 0.05% or less (including 0%).

Cu is generally often added to Al—Zn—Mg alloys, because this element has the functions of allowing the alloys to have better SCC resistance and to have higher strength. Zr, Mn, Cr, and Sc are often added to the alloys, because these elements contribute to refinement of grains and resulting higher strength of the ingots and of the final product sheet. Cu, Zr, Mn, Cr, and Sc, when added to such regular Al—Zn—Mg alloys, may highly probably cause the resulting sheets to fail to have a high [surface part Cube] as specified in the present invention, where the [surface part Cube] is the area percentage of grains with Cube orientation in the surface part. These elements, when added to such regular Al—Zn—Mg alloys, may also highly probably cause the resulting sheets to fail to have a [surface part S] controlled within the range specified in the present invention, where the [surface part S] is the area percentage of grains with S orientation in the surface part.

Either one or both of Ag in a content of 0.01% to 0.2% and Sn in a content of 0.001% to 0.1%

Silver (Ag) and tin (Sn) may be contained selectively as needed, because these elements effectively allow aged precipitates to precipitate densely and finely, and promotively allow the aluminum alloy sheet to have higher strength, where the aged precipitates contribute to higher strength through artificial aging after forming/working into structural components. These elements, when either one or both of them are contained in a Ag content of less than 0.01% and in a Sn content of less than 0.001%, may less effectively allow the sheet to have higher strength. In contrast, Sn and Ag, if either one or both of them are contained in excessively high contents, may contrarily cause deterioration in properties such as rolling properties and weldability, may offer saturated effects of improving strength, and for Ag, such use in an excessively high content merely causes high cost. Accordingly, Ag may be added in a content of 0.01% to 0.2%, and Sn may be added in a content of 0.001% to 0.1%.

Ti: 0.001% to 0.1%

Titanium (Ti), as with boron (B), acts as an impurity in a rolled sheet, but effectively refines grains in aluminum alloy ingots. These elements are therefore allowed to be contained within ranges specified as 7xxx-series alloys by JIS. Ti, if present in a content of less than 0.001%, may fail to effectively contribute to grain refinement. In contrast, Ti, if present in a content of greater than 0.1%, may form coarse compounds and may cause the sheet to have inferior mechanical properties. To eliminate or minimize these, the upper limit of the Ti content is 0.1%, and preferably 0.05% or less. In combination with Ti, boron (B) may be contained in a content up to 0.03%. Boron, if present in a content of greater than 0.03%, may form coarse compounds and may cause the sheet to have inferior mechanical properties.

Other Elements

Other elements than those mentioned above are unavoidable impurities. These impurity elements may be mixed into the sheet typically upon the use of aluminum alloy scrap in combination with or instead of pure aluminum ingots as raw materials to be melted. On the assumption (in allowance) typically of such mixing, the impurity elements may be present in the sheet within ranges specified as 7xxx-series alloys by JIS. For example, Fe in a content of 0.5% or less and Si in a content of 0.5% or less do not adversely affect the properties of the aluminum alloy sheet according to the present invention and may be present as unavoidable impurities.

Microstructure

As a precondition, the7xxx-series aluminum alloy sheet according to the present invention is in common in chemical composition and in most of production steps with conventional 7xxx-series aluminum alloy sheets and their production methods (common rolling process). The aluminum alloy sheet according to the present invention is therefore in common with the conventional 7xxx-series aluminum alloy sheets also in that fine, nano-level-sized precipitates are present in a large number in grains as sheet microstructure and make a foundation to offer satisfactory basic properties such as strength and corrosion resistance. These fine, nano-level-sized precipitates are fine dispersoids corresponding to the Mg—Zn intermetallic compounds (having a chemical composition typically of MgZn₂) and further including one or more elements corresponding to the chemical composition.

Texture

On the premise of having the chemical composition as described above, the 7xxx-series aluminum alloy sheet according to the present invention is controlled in microstructure so as to have better levels of properties such as shock absorption (crushing properties) upon automobile collision, strength, and corrosion resistance, where the shock absorption is evaluated by the VDA bend test.

Specifically, a surface part of the sheet is controlled to have an equiaxial recrystallized microstructure having an average grain size of 50 μm or less, where the surface part is a part or portion ranging from the sheet surface to a depth of 15% of the sheet thickness. Assume that the surface part coarsens to have an average grain size of greater than 50 μm; or the surface part has not the equiaxial recrystallized microstructure, but an as-deformed microstructure, which is an elongate deformed microstructure elongating in the sheet rolling direction. The sheet in this case may have inferior shock absorption (crushing properties), because grains with Cube orientation, which contribute to better VDA bendability, do not develop.

As used herein the term “depth of 15% of the sheet thickness” (from the sheet surface) refers to the depth from the outermost surface of aluminum matrix, of a sheet resulting from removing an oxide layer from the sheet (test sample) surface by polishing.

Of course, not only the surface part, but also other portions toward the core of the sheet are preferably controlled to have such an equiaxial fine recrystallized microstructure having an average grain size of 50 μm or less. However, it is difficult to allow the microstructure of sheet internal portions such as the thickness-central part to be an equiaxial fine recrystallized microstructure having an average grain size of 50 μm or less when the sheet is production by a common procedure, particularly typically when the sheet has a large thickness.

The shock absorption (crushing properties) is significantly affected not by the state of the microstructure of sheet internal portions such as the thickness-central part, but by the microstructure of grains with Cube orientation in the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness. On the basis of this, the present invention specifies the texture of the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness.

The surface part, if having a thickness of less than 15% of the sheet thickness from the sheet surface, less effectively contributes to better shock absorption, even when the surface part meets after-mentioned conditions on the area percentages of grains with Cube orientation and S orientation. This is because the surface part in this case is excessively thin and occupies, for example, only the surface of the sheet. The surface part having the specific texture, if ranging from the sheet surface to a depth greater than 15% of the sheet thickness, occupies an excessively large part of the sheet, and this causes the sheet central part, which ensures strength at certain level, to be excessively thin and causes the sheet as a whole to fail to surely have high strength.

Differentiation of Textures of Surface Part and Thickness-Central Part

In addition to allowing the sheet surface part to have a fine, equiaxial recrystallized microstructure as described above, the present invention further specifies following conditions. The sheet surface part is controlled to have a texture mainly including grains with Cube orientation, which more excel in shock absorption (crushing properties) than in strength; and the sheet thickness-central part is controlled to have a texture mainly including grains with S orientation, which more excel in strength than in shock absorption (crushing properties).

Specifically, the present invention allows the sheet surface part to include grains with Cube orientation, which contribute to the shock absorption (crushing properties), in an absolute amount at a certain level and in a larger relative amount as compared with grains with Cube orientation in the thickness-central part. This configuration allows the sheet to have better shock absorption (crushing properties).

In combination with this, the thickness-central part is allowed to include grains with S orientation, which are contribute to better strength, in an absolute amount at a certain level and in a larger area percentage as compared with the area percentage of grains with S orientation in the surface part. This configuration allows the sheet to surely have strength at a certain level.

As described above, the present invention controls the surface part and the thickness-central part in the sheet to have different textures that are optimal respectively for shock absorption and for strength, and allows the sheet to have a complex microstructure differing in the sheet thickness direction, in which the sur ace part and the thickness-central part differ from each other in texture and in action.

Area Percentages of Cube Orientation and S Orientation in Surface Part and Thickness-Central Part

Specifically, the present invention defines as follows. Of grains in the sheet surface part ranging from the sheet surface to a depth of 15% of the sheet thickness, the area percentage of grains with Cube orientation is defined as a [surface part Cube], and the area percentage of grains with S orientation is defined as a [surface part S]. Likewise, of grains in the sheet thickness-central part, the area percentage of grains with Cube orientation is defined as a [thickness-central part Cube], and the area percentage of grains with S orientation is defined as a [thickness-central part S].

Of grains in the surface part ranging to a depth of 15% of the sheet thickness, the [surface part Cube] is controlled to be 10% or more, and the [surface part S] is controlled to be 10% to 40%.

In addition, the ratio [surface part Cube]/[thickness-central part Cube], which is the ratio of the [surface part Cube] to the [thickness-central part Cube], is controlled to be greater than 1.0, and the ratio [surface part S]/[thickness-central part S], which is the ratio of the [surface part S] to the [thickness-central part S], is controlled to be less than 1.0.

A sheet produced by a common procedure has a ratio of the [surface part Cube] to [thickness-central part Cube] of 1.0 and a ratio of the [surface part S] to [thickness-central part S] of 1.0. In this sheet, the surface part and the thickness-central part have an approximately identical texture to each other.

The upper limit of the [surface part Cube] is about 80% in consideration of a production limit. From this viewpoint, the [surface part Cube] preferably falls within the range of 10% to 80%.

Control of the [surface part Cube] as above allows the sheet to have such a microstructure as to uniformly deform without local strain concentration when the sheet receives the strain in the VDA bend test, even when the sheet is a 7xxx-series aluminum alloy sheet produced by a common procedure. This allows the sheet to have properties with high shock absorption (crushing properties) upon automobile collision, even when having a high strength in terms of 0.2% yield strength of typically 350 MPa or more, where such high strength is obtained by the control of the [thickness-central part S].

Assume that a sheet has a low [surface part Cube] of less than 10%, or has an excessively high [surface part S] of greater than 40%, or a ratio of the [surface part Cube] to the [thickness-central part Cube] of 1.0 or less, namely, an excessively low [surface part Cube]. This sheet offers lower VDA bendability and lower shock absorption (crushing properties) upon automobile collision. Also assume that a sheet has a ratio of the [surface part Cube] to the [thickness-central part Cube] of 1.0 or less, namely, an excessively high [thickness-central part Cube]. This sheet has lower strength due to an excessively low [thickness-central part S] as described below.

In contrast, assume that a sheet has an excessively low [surface part S] of less than 10%, or has a ratio of the [surface part S] to the [thickness-central part S] of 1.0 or more, namely, an excessively low [thickness-central part S], or has an excessively high [thickness-central part S]. This sheet has lower strength.

For still better VDA bendability and strength, the sheet preferably has a ratio of the [surface part Cube] to the [thickness-central part Cube] of 1.2 or more and a ratio of the [surface part S] to the [thickness-central part S] of 0.8 or less; and more preferably has a ratio of the [surface part Cube] to the [thickness-central part Cube] of 1.3 or more and a ratio of the [surface part S] to the [thickness-central part S] of 0.7 or less.

As long as meeting these conditions or relationships on Cube orientation and S orientation, the sheet may further include grains with other orientations. Non-limiting examples of the other orientations include CR orientation, Brass orientation, Cu orientation, Goss orientation, Rotated-Goss orientation, S orientation, B/G orientation, B/S orientation, and P orientation. It is difficult to eliminate grains with the other orientations, also due to limits of production by a common procedure.

Increase in the [surface part Cube] requires reduction in amount of a residual rolling texture in recrystallization, where the recrystallization occurs upon solution treatment of a sheet such as a cold-rolled sheet or a hot-rolled sheet. As a rough reference of this, the [surface part S] is minimized. The sheet, if including a large amount of the rolling texture and having a high [surface part S], may have lower shock absorption (crushing properties) upon automobile collision.

Texture Measurement

The average grain size and the area percentages of grains with the orientations, as specified in the present invention, are each measured by electron backscatter pattern (EBSP; or electron backscatter diffraction (EBSD)) analysis. More specifically, a surface part and a thickness-central part are independently sampled from a cross section in the transverse direction of a cold-rolled sheet or a hot-rolled sheet after the solution treatment (T4 sheet), where the surface part ranges from the sheet surface to a depth of 15% of the sheet thickness. The sampled surface part and the thickness-central part are subjected sequentially to mechanical polishing, buffing, and electropolishing and yield samples having modified surfaces. The crystal orientations and the grain size of the surface part sample and the thickness-central part sample are independently measured by EBSP analysis using a scanning electron microscope (SEM) or a field emission scanning electron microscope (FESEM). Thus, the [surface part Cube] and the [surface part S] in the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness, and the [thickness-central part Cube] and the [thickness-central part S] in the sheet thickness-central part are measured.

EBSP measurement and analysis may be performed using an EBSP system OIM (supplied by TSL) or CHANNEL 5 (supplied by OXFORD). The sheet microstructure is measured at positions as with common measurement positions for such microstructures. Specifically, five measurement test specimens are sampled at any five measurement positions in a cross section in the sheet transverse direction. The measured values in the surface part and in the thickness-central part are individually averaged, and these are defined as the average grain size and the area percentages of grains with the orientations, as specified in the present invention.

The SEM/EBSP analysis is generally widely used as a texture measurement technique and is a crystal orientation analysis technique using a scanning electron microscope (SEM) or a field emission scanning electron microscope (FESEM) equipped with an electron backscattering (scattered) pattern (EBSP) analysis system. Advantageously, this measurement technique has higher resolution and thereby has higher measurement precision as compared with other texture measurement techniques and also enables simultaneous, high-precision measurement of the average grain size at the same measurement position of the sheet.

In the SEM/EBSP analysis, an aluminum alloy sheet sample is placed in the lens barrel of the SEM or FESEM (FE-SEM) and is irradiated with electron beams to project an EBSP on the screen. An image of this is taken with a highly sensitive camera and captured as an image into a computer. The image is analyzed in the computer and compared with patterns as a result of simulation using known crystal systems, to determine crystal orientations. The calculated crystal orientations are recorded as three-dimensional Eulerian angles typically with position coordinates (x, y). This process is automatically performed on all measurement points, and gives crystal orientation data at several tens of thousands to several hundreds of thousands of points upon the completion of measurement. The crystal orientation analysis technique using the SEM or FESEM equipped with the EBSP analysis system is described in detail typically in Research and Development, Kobe Steel Engineering Reports, Vol. 52, No. 2 (September 2002) pp. 66-70.

An aluminum alloy sheet generally has a texture including many orientation components (grains with the orientations) as mentioned below and having crystal planes corresponding to the orientation components. In general, an aluminum alloy rolled sheet has a texture mainly including Cube orientation, Goss orientation, Brass orientation, S orientation, and Copper orientation. The way to form these textures varies depending on working and on heat treatment method even in the same crystal system. The textures of a sheet formed via rolling may be represented by the rolling plane {hkl} and the sheet rolling direction <uvw>. According to this indication system, the orientations are expressed as follows.

-   Cube orientation {001}<100> -   Goss orientation {011}<100> -   Brass orientation (B orientation) {011}<211> -   Cu orientation (Copper orientation) {112}<111> -   S orientation {123}<634> -   B/G orientation {011}<511> -   B/S orientation {168}<211> -   P orientation {011}<111>

Basically in the present invention, grain boundaries having a deviation (tilt angle) in orientation of less than ±10° from these crystal planes are considered to belong to the same crystal plane (orientation component). In addition, the boundary of adjacent grains with misorientation (tilt angle) being 5° or more is defined as a grain boundary.

The [surface part Cube] and [surface part S] in the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness, and the [thickness-central part Cube] and [thickness-central part S] in the sheet thickness-central part are individually calculated by the crystal orientation analysis technique using the SEM or FESEM equipped with the EBSP analysis system.

In this process, the area percentages of the orientations as specified in the present invention are calculated with the total area of the crystal orientations from Cube orientation to P orientation (all crystal orientations) being defined as 100.

The average grain size is also measured and calculated at grain boundaries with tilt angles of 5° or more. In other words, the average grain size is calculated in the present invention by an expression below, provided that a deviation in the orientation of less than ±5° is defined to belong to the same grain, and the boundary of adjacent grains with misorientation (tilt angle) being 5° or more is defined as a grain boundary in the present invention. The expression is expressed as follows:

Average grain size=(Σx)/n

where n represents the number of measured grains; and x represents the grain size of each grain.

Production Method

The 7xxx-series aluminum alloy sheet according to the present invention is produced by a common procedure, where the aluminum alloy sheet is a cold-rolled sheet prepared by subjecting ingots sequentially to soaking, hot rolling, and cold rolling, and is further subjected to a heat treatment such as solution treatment. Specifically, an aluminum alloy hot-rolled sheet having a thickness of about 2 to about 10 mm is prepared via a common production process including casting, homogenization (soaking), and hot rolling. Next, the hot-rolled sheet is subjected to cold rolling to give a cold-rolled sheet having a thickness of 3 mm or less.

Accordingly, the 7xxx-series aluminum alloy sheet according to the present invention is not produced by a special production method or special rolling method, typically in which hot rolling is omitted by performing cold rolling after continuous casting into a thin sheet typically by twin-roll technique, or in which warm rolling is performed. However, to provide the textures specified in the present invention, the soaking and solution treatment are performed under conditions different from those in the process by a common procedure, as described below.

Melting, and Casting Cooling Rate

Initially, in a melting and casting process, an aluminum alloy is melted and adjusted so as to have a chemical composition within the 7xxx-series range to give a molten aluminum alloy, and the molten aluminum alloy is cast by a common melt casting technique selected as appropriate typically from continuous casting and semicontinuous casting (direct chill casting (DC casting)).

Homogenization

Next, the cast aluminum alloy ingots are subjected to homogenization before hot rolling. The homogenization (soaking) is performed in order to homogenize the microstructure, namely, to eliminate or minimize grain segregation in the ingot microstructure. However, the soaking in the present invention is performed not as common single soaking, but as double soaking or two-stage soaking so as to give the textures specified in the present invention, because such soaking also significantly affects the formation of textures.

In the double soaking, the work after first soaking is once cooled down to a temperature of 200° C. or lower including room temperature, further reheated, and held at that temperature for a predetermined time, followed by start of hot rolling. In contrast, in the two-stage soaking, the work after first soaking is cooled down to a temperature not being 200° C. or lower, but being higher than 200° C., and held at that temperature, followed by hot rolling start as intact at that temperature or after reheated to a higher temperature.

The first or first-stage soaking is performed under conditions selected as appropriate at a temperature of from 400° C. to lower than the melting point, for a holding time of 2 hours or longer.

After the first soaking, the work is cooled once down to a temperature of 200° C. or lower including room temperature (for double soaking), or is cooled once down to a temperature higher than 200° C. (for two-stage soaking). This cooling is performed as rapid cooling at a cooling rate of 30° C./hr or more, and preferably 40° C./hr or more, in common in both double soaking and two-stage soaking.

Cooling at such a high cooling rate as above restrains precipitation of coarse dispersed particles during cooling, and thereby enhances growth of a rolling texture upon cold rolling. This allows the [surface part Cube] to increase (grow), and controls the [surface part S] within an appropriate proportion in recrystallization, which occurs upon solution treatment.

The aluminum alloy sheet can have the textures specified in the present invention by performing cooling after first soaking under the conditions as above in double soaking or two-stage soaking, in combination with performing cold rolling and solution treatment under conditions as mentioned later.

In contrast, soaking, if performed under conditions out of the cooling conditions or if performed as common single soaking, may highly possibly fail to give the textures specified in the present invention, even when the cold rolling and solution treatment are performed under conditions within preferred ranges as mentioned later.

The second or second-stage soaking may be performed under conditions as follows. The reheating temperature may be selected within the range of from the hot rolling start temperature to 500° C., and the holding time may be selected within the range of 2 hours or longer. The ingots after first soaking and cooling may be reheated and then cooled down to the hot rolling start temperature, or reheated up to the hot rolling start temperature and held at a temperature adjacent to that temperature. Alternatively, the ingots after first-stage soaking may be cooled down to the hot rolling start temperature and held at a temperature adjacent to that temperature. The second or second-stage soaking is preferably performed at a lower temperature as compared with the first or first-stage soaking temperature.

Hot Rolling

Hot rolling, if started at a temperature of higher than the solidus temperature, causes burning, and this impedes the hot rolling itself. Hot rolling, if started at a temperature of lower than 350° C., requires an excessively high load during hot rolling, and this impedes the hot rolling itself. To eliminate or minimize these, hot rolling is performed at a hot rolling start temperature of from 350° C. to the solidus temperature and gives a hot-rolled sheet having a thickness of about 2 to about 10 mm. Annealing before cold rolling of the hot-rolled sheet is not always necessary, but may be performed.

Cold Rolling

In cold rolling, the hot-rolled sheet is rolled into a sold-rolled sheet (including a coil) having a desired final thickness typically of about 1 to about 5 mm when the sheet is used for automobile structural components. Assume that a heat treatment (rough annealing) between hot rolling and cold rolling is performed, and/or process annealing during cold rolling is performed. In this case, the cold rolling is performed at a cold rolling reduction (total cold rolling reduction) of 50% or more so as to form the textures specified in the present invention. Cold rolling, if performed at a cold rolling reduction of less than 50%, may highly possibly fail to give the textures specified in the present invention even when the soaking and the downstream solution treatment are performed under preferred conditions. In contrast, the upper limit of the cold rolling reduction is determined on the basis of production limits and is about 98%.

The number of the cold rolling steps is freely selected depending on the relationship between the hot-rolled sheet thickness and the cold-rolled sheet final thickness, and the number of passes of the sheet (coil) to a cold rolling mill per cold rolling step is also freely selected.

The heat treatment and the process annealing during cold rolling are performed at temperatures in the range of 380° C. to 500° C., for an appropriate time according to threading conditions in a continuous furnace or batch furnace to be used. Cooling after the process annealing is preferably performed as forced cooling, such as air cooling with a fan. These annealing processes, when performed as continuous annealing, are preferably performed at an end-point temperature of 450° C. or higher. This is preferred from the viewpoint of surely providing solid solution in the downstream solution treatment. The annealing processes, when performed as batch annealing, are preferably performed at an end-point temperature of 420° C. or lower. This is because the annealing processes, if performed at a high end-point temperature, may cause larger amounts of precipitates during cooling, due to a low cooling rate.

Solution Treatment

After the cold rolling, a solution treatment is performed as a heat treatment. The solution treatment may be performed by heating and cooling the work in a common continuous heat treatment line, without limitation. However, the solution treatment is desirably performed at a temperature of 450° C. to 550° C. so as to give sufficient amounts of solute elements and to perform grain refinement.

Heating (temperature rise) in the solution treatment is preferably performed at three different rates in three individual stages. Initially, heating (temperature rise) is performed at a rate of from 0.001° C./s to 10° C./s on average, in a region at sheet temperatures of 300° C. or lower. Next, heating (temperature rise) is performed at a rate of from 10° C./s to 100° C./s on average, in a region at sheet temperatures of 300° C. to 450° C. Then, heating (temperature rise) is performed at a rate of from 0.001° C./s to 10° C./s on average, in a region at sheet temperatures of from 450° C. to the solidus temperature.

Recrystallization during heating (temperature rise) starts at a lower temperature in the sheet surface part than in the sheet thickness-central part (central part). Accordingly, heating (temperature rise) at the low rate in the region at sheet temperatures of 300° C. or lower allows the [surface part Cube] to preferentially increase (grow) and can control the [surface part S] within an appropriate proportion.

Heating (temperature rise) at the high rate in the region at sheet temperatures of 300° C. to 450° C. allows the [thickness-central part S] to preferentially increase (grow) and can restrain recrystallized grains from coarsening.

Heating (temperature rise) at the low rate in the region at sheet temperatures of from 450° C. to the solidus temperature allows the strengthening elements to surely undergo solid solution.

The heating is hardly performed at an average heating rate of greater than 100° C./s due to limitations of facility capacity of the solution treatment furnace.

Cooling (temperature fall) after the solution treatment is preferably performed at an average cooling rate of 10° C./s or more so as to restrain precipitation of coarse dispersed particles during cooling. The cooling after the solution treatment is therefore preferably performed as forged cooling, or as quenching directly in water or hot water at a temperature of room temperature to 100° C. The forced cooling may be performed using a forced cooling means such as air cooling typically with a fan; and water cooling means such as mist, sprays (aerosols), and immersion.

The solution treatment is basically performed only once. However, typically when natural aging at room temperature proceeds excessively, another solution treatment may be performed under the preferred conditions to once cancel the excessively proceeding natural aging at room temperature. This is preferred to surely give formability typically into automobile members.

The aluminum alloy sheet according to the present invention is used as a material, subjected to press forming and/or working such as burring and bore expanding, and formed into structural components typically for automobiles, bicycles, and railroad vehicles. The structural components after forming and working may be subjected to artificial aging as needed, so as to have higher strength. The forming and working is performed before artificial aging because of ensuring formability.

Artificial Aging

The artificial aging may be performed under common artificial aging conditions (T6 or T7). The conditions such as temperature and time are freely determined according typically to the desired strength, the material7xxx-series aluminum alloy sheet strength, or how much the natural aging at room temperature proceeds. For example, a single-stage temper aging may be performed as temper aging at 100° C. to 150° C. for 12 to 36 hours (including the over-aging region). A two-stage temper aging may be performed under selected conditions, in which a first-stage heat treatment is performed at a temperature of from70° C. to 100° C. for 2 hours or longer, and a second-stage heat treatment is performed at a temperature of from 100° C. to 170° C. for 5 hours or longer (including the over-aging region).

EXAMPLES

Cold-rolled sheets were produced from 7xxx-series aluminum alloys having the chemical compositions given in Table 1, under different conditions as given in Table 2 so as to have different textures. The cold-rolled sheets were evaluated on mechanical properties such as strength; and on shock absorption (crushing properties), where the shock absorption is evaluated by VDA bend tests. The results of them are presented in Table 3.

The cold-rolled sheet textures were controlled mainly by varying soaking conditions and solution treatment conditions as given in Table 2. Specifically, in common to each sample, 7xxx-series molten aluminum alloys having the chemical compositions given in Table 1 were subjected to direct chill casting (DC casting) to give ingots, the ingots were subjected to soaking and hot rolling under the soaking conditions and at the hot rolling start temperatures given in Table 2 and yielded hot-rolled sheets having thicknesses of 3 mm to 25 mm. These hot-rolled sheets, in common to each sample, were heat-treated (annealed) by holding at 500° C. for 30 seconds and subsequent forced-wind-cooling, were cold-rolled with process annealing being performed under the conditions given in Table 2, and yielded cold-rolled sheets having a thickness of 2 mm in common. In Table 1, the symbol “-” in content of an element indicates that the content of the element is lass than the detection limit.

In cold rolling, in common to each sample, three cold rolling steps were performed, and three passes were performed per cold rolling step. Process annealing between the cold rolling steps was performed as follows. When a continuous annealing furnace was employed, heating was performed at a rate of temperature rise of 200° C./min, and cooling was performed as air cooling with a fan. When a batch annealing furnace was employed, heating and cooling were performed at a rate of temperature rise/drop of 30° C./hr. As process annealing in Table 2, all the samples underwent continuous annealing, except Example 2, which underwent batch annealing.

These cold-rolled sheets were subjected to solution treatments performed at efferent average rates of temperature rise in three stages divided by the sheet temperature ranges, as given in Table 2. The solution treatments were performed under conditions of the holding temperature, holding time, and average cooling rate also given in Table 2 to give T4 sheets.

The T4 sheets were aged at room temperature for one week, from which test samples were sampled. Of the test samples, the textures and fine precipitates (as reference) were examined, and mechanical properties were investigated by tensile tests as mentioned below. The results of them are presented in Table 3.

Textures and Average Grain Size

The textures and average grain sizes of sheet-like test specimens sampled from the T4 sheets were measured by performing electron backscatter diffraction pattern (EBSD, EBSP) measurement on the rolling planes of the surface part and the thickness-central part, to determine area percentages and grain sizes (equivalent circle diameters) of grains with individual orientation components.

Specifically, the measurement was performed by the measurement method on the rolling planes of the surface part and the thickness-central part of the sheet-like test specimens sampled from the T4 sheets. The measurement was performed using the SEM (JEOL JSM 6500F, supplied by JEOL Ltd) equipped with the EBSP Measurement-Analysis System (OIM, supplied by TSL). In common to each sample, the measurement was performed on each five test specimens sampled at any positions in the surface part and in the thickness-central part in the sheet rolling direction, and each five measurements were averaged. The measurement of each test specimen was performed in an area of 1000 μm by 1000 μm at a measurement step of 1 μm in common. The sheet rolling plane in the surface part was a rolling plane at a position from the test specimen surface to a depth of up to 15% in the sheet thiclmess direction, where an oxide layer had been removed from the test specimen surface. The sheet rolling plane in the thickness-central part was a rolling plane at a position (depth) of 50% in the sheet thickness direction.

Fine Precipitates

In common to each sample, there were prepared cross sections of portions at a depth of one-half the thickness from the surface of the sheet-like test specimens sampled from the T4 sheets, namely, the portions are each a center in the sheet thickness direction. The cross sections were observed using a transmission electron microscope at 300000-fold magnification, and the average number density (number per square micrometer) of precipitates having a size of 2.0 to 20 nm in grains was measured as reference. The measurement was performed on five test specimens per sample, and the number densities of precipitates having a size of 2.0 to 20 nm in grains were individually determined and averaged to give an average number density. All the examples (inventive examples) had average number densities of precipitates having a size of 2.0 to 20 nm of from 2×10⁴ to 9×10⁴ per square micrometer. The sizes of the precipitates were measured in terms of equivalent circle diameter, which is the diameter of a circle having an area equivalent to the area of the precipitate in question.

Separately, the T4 sheets were aged at room temperature for one week, then artificially aged in common under two-stage conditions of at 90° C. for 3 hours and at 140° C. for 8 hours, and yielded T6 sheets. The artificial aging was performed as a T6 treatment and simulated artificial aging after forming/working into structural components. Sheet-like test specimens were sampled from the central parts of the T6 aluminum alloy sheets, on which mechanical properties and corrosion resistance were examined in the following manner. The results of them are also presented in Table 3.

Mechanical Properties

In common to each sample, the sheet-like test specimens sampled from the T6 sheets or the T4 sheets were processed into JIS No. 5 test specimens, subjected to tensile tests at room temperature so that the tensile direction was in parallel with the sheet rolling direction, and tensile strength (MPa) and 0.2% yield strength (MPa) were measured. The tensile tests at room temperature were performed in conformity with JIS 2241:1980 at room temperature of 20° C. at a gauge length of 50 mm and a constant tensile speed of 5 mm/min. until the test specimen ruptured.

Shock Absorption

Bend tests to evaluate shock absorption were performed as VDA bend tests in conformity with “VDA 238-100 Plate bending test for metallic materials” standardized by the German Automotive Industry Association (VDA). The testing method is illustrated as a perspective view in FIG. 1.

Initially, a sheet-like test specimen sampled from the T6 sheet is placed horizontally and equally in length on both sides on two rolls, as indicated by the dotted lines in FIG. 1, where the two rolls are disposed in parallel with each other with a roll gap.

Specifically, the sheet-like test specimen sampled from the T6 sheet is placed horizontally and equally in length on both sides on the two rolls, so that the sheet rolling direction is perpendicular to the extending direction of a sheet-like bending jig placed vertically above the test specimen and so that the central part of the test specimen is positioned at the center of the roll gap.

The bending jig is pressed against the central part of the sheet-like test specimen to put a load on the test specimen, thereby bends the sheet-like test specimen by pulsing (by pressing) toward the narrow roll gap, and presses the central part of the bent and deformed sheet-like test specimen into the narrow roll gap.

In this process, a bending angle (in degree) at the time when the load F, which is put by the bending jig from above, becomes maximum is measured, and the shock absorption is evaluated on the basis of magnitude of the bending angle, where the bending angle is the outer bending angle of the central part of the sheet-like test specimen. With an increasing bending angle, the sheet-like test specimen remains in a bending deformation state without crushing during bending and offers higher shock absorption (crushing properties).

With reference to the symbols in FIG. 1, the testing conditions of the VDA bend tests are as follows. The sheet-like test specimen has a square shape of a width b of 60 mm by a length l of 60 mm. The two rolls each have a diameter D of 30 mm with a roll gap L of 4 mm, which is 2.0 times as much as the thickness of the sheet-like test specimen. At the time when the load F becomes maximum, the central part of the sheet-like test specimen is pressed into the roll gap at a depths.

The sheet-like bending jig has a tapered narrow shape so that the lower side, which is pressed against the central part of the sheet-like test specimen, has a tip (lower end) radius of 0.2 mm.

The bend tests were performed on three sheet-like test specimens (three times) per sample, and an average of the resulting three bending angles was defined as the bending angle (in degree).

Grain-Boundary Corrosion Susceptibility

Corrosion resistance evaluation, which leads to SCC resistance evaluation, was performed as grain-boundary corrosion susceptibility tests in conformity with the old JIS W 1103 standard on the sheet-like test specimens (three test specimens) after the artificial aging.

The tests were performed under conditions as follows. Each test specimen was immersed sequentially in a nitric acid aqueous solution (30 mass percent) at room temperature for 1 minute, in a sodium hydroxide aqueous solution (5 mass percent) at 40° C. for 20 seconds, and in a nitric acid aqueous solution (30 mass percent) at room temperature for 1 minute to wash the surface of the test specimen. A current at a current density of 1 mA/cm² was applied for 24 hours while the test specimen was immersed in a sodium chloride aqueous solution (5 mass percent). The test specimen was then retrieved, a cross section of which was cut out and polished. The depth of corrosion from the test specimen surface was measured using an optical microscope at 100-fold magnification. A sample having a corrosion depth of 200 μm or less was evaluated as undergoing slight corrosion and indicated as “◯” (good); whereas a sample having a corrosion depth of greater than 200 μm was evaluated as undergoing significant corrosion and indicated as “×” (poor).

As obvious from Tables 1 to 3, the examples have aluminum alloy chemical compositions within the ranges specified in the present invention and are produced under soaking conditions and cold rolling conditions within the preferred ranges specified in the present invention. As a result, the T4 sheets have microstructures as follows. Specifically, the T4 sheets each have an equiaxial recrystallized microstructure having an average grain size of 40 μm or less in the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness, and each have a [surface part Cube] of 10% or more and a [surface part S] of 10% to 40%. In addition, the T4 sheets each have a ratio of the [surface part Cube] to the [thickness-central part Cube] of greater than 1.0 and a ratio of the [surface part S] to the [thickness-central part S] of less than 1.0.

The T6 sheets, as a result, have VDA bending angles at a high level of 67° to 55° at a strength level in terms of 02% yield strength of 358 to 391 MPa and have VDA bending angles also at a high level of 44° to 40° even at a strength level in terms of 0.2% yield strength of 425 to 446 MPa. Thus, the T6 sheets are found to combine strength and shock absorption (crushing properties) with each other and to also have excellent corrosion resistance.

Examples 1 and 2 in Tables 2 and 3 are good examples having Mg and Zn contents in good balance so as to have necessary strength, in other words, so as to control strength. Example 2 has a lower Zn content as compared with Example 1 as presented in Table 1, but has a higher 0.2% yield strength as compared with Example 1 as presented in Table 3. This is because Example 2 ensures strength by balancing the Mg and Zn contents with each other and by having a higher Mg content as compared with Example 1, so as to supplement the lower Zn content. In contrast, Example 1 has a higher Zn content, but has a 0.2% yield strength controlled to be low by balancing the Mg and Zn contents with each other and by having a lower Mg content as compared with Example 2.

In contrast, the comparative examples in Tables 2 and 3 have alloy chemical compositions out of the ranges specified in the present invention, as presented in Table 1, or are produced under conditions out of any of the preferred soaking conditions and cold rolling conditions in spite of having alloy chemical compositions within the ranges specified in the present invention. As a result, these comparative examples fail to have desired textures, or, even when having desired textures, have lower VDA bending angles for their strengths.

Comparative Examples 9 to 15 employ Alloy Nos. 1 and 3 in Table 1, which are alloys meeting the conditions specified in the present invention. These comparative examples, however, are produced under conditions out of any of the preferred conditions. As presented in Table 2, for example, Comparative Example 10 has undergone only single soaking; Comparative Examples 10 to 12 have undergone cooling at excessively low cooling rates after the first soaking; Comparative Examples 9 and 10 have undergone cold rolling at excessively low cold rolling reductions; and Comparative Examples 9, 11, 12, 13, 14, and 15 have undergone solution treatments at average rates of temperature rise out of the preferred ranges in the three temperature regions.

These comparative examples have microstructures of the T4 sheets as follows. Specifically, these comparative examples each have an equiaxial recrystallized microstructure having an average grain size of 40 μm or less in the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness, but have: a [surface part Cube] of lass than 10% (Comparative Example 9); a ratio of the [smface part Cube] to the [thickness-central part Cube] of 1.0 or less (Comparative Examples 9 to 11, 13, and 15); and/or a ratio of the [surface part S] to the [thickness-central part S] of 1.0 or more (Comparative Examples 9 to 15).

As a result, the comparative examples have VDA bending angles of the T6 sheets at a significantly lower level of 42° to 37° at a strength level in terms of 0.2% yield strength of 346 to 375 MPa, as compared with the level of the VDA bending angles of the examples at the same strength level. These results demonstrate that the comparative examples fail to combine strength and shock absorption (crushing properties) with each other.

Comparative Examples 16 to 23 are produced under conditions within the preferred ranges, except Comparative Examples 19 and 20, but employ Alloy Nos. 9 to 16 in Table 1, and have chemical compositions out of the chemical composition ranges specified in the present invention. For example, Alloy No. 9 has an excessively low Zn content, Alloy No. 10 has an excessively low Mg content, and Alloy Nos. 11 to 16 have excessively high contents of one or more of Cu, Zr, Mn, Cr, and Sc.

In addition, Comparative Example 19 has undergone only single soaking; Comparative Examples 19 and 20 have undergone cold rolling at excessively low cold rolling reductions; and Comparative Example 20 has undergone a solution treatment at average rates of temperature rise out of the preferred ranges in the three temperature regions, and has undergone cooling after the solution treatment at an average cooling rate out lithe preferred range.

These comparative examples therefore have microstructures of the T4 sheets as follows. Specifically, as presented in Table 3, these comparative examples have microstructures having an average grain size of greater than 40 μm in the surface part ranging from the sheet surface to a depth of 15% of the sheet thickness (Comparative Examples 16 to 18), have a [surface part Cube] of less than 10% (Comparative Examples 18 to 23), have a ratio of the [surface part Cube] to the [thickness-central part Cube] of 1.0 or less (Comparative Examples 16 and 18 to 23), and/or have a ratio of the [surface part S] to the [thickness-central part S] of 1.0 or more (Comparative Examples 16 to 23).

As a result, these comparative examples have VDA bending angles of the T6 sheets at a low level of 53° to 32° at a strength level in terms of 0.2% yield strength of 319 to 370 MPa and have VDA bending angles at a low level of 38° to 32° at a strength level in terms of 0.2% yield strength of 428 to 471 MPa, both of which VDA bending angle levels are significantly lower as compared with the VDA angle level of the examples at the same strength level. These demonstrate that the comparative examples fail to combine strength and shock absorption (crushing properties) with each other.

In addition, Comparative Example 16 has an excessively low strength, and Comparative Example 18 has excessively low corrosion resistance.

These results support critical significance of the conditions specified in the present invention, so as to allow the aluminum alloy sheets according to the present invention to have shock absorption (crushing properties), strength, and corrosion resistance all at satisfactory levels, where the shock absorption is evaluated by the VDA bend tests.

TABLE 1 Aluminum alloy chemical composition in mass percent (with the remainder being Al) Category Number Zn Mg Cu Zr Mn Cr Sc Ag Sn Si Fe Ti Inventive 1 6.7 1.4 0.16 — — — — — — 0.08 0.10 0.02 alloys 2 3.7 4.1 0.20 — — — — — — 0.10 0.08 0.02 3 6.4 1.4 — — — — — — — — — — 4 2.5 4.3 — 0.08 0.05 0.05 — — — — — — 5 8.3 0.8 — — — 0.10 0.02 — 0.07 — — — 6 4.8 3.0 0.16 0.04 0.04 0.10 0.03 — — 0.08 0.10 0.02 7 4.8 3.1 — — — — — 0.02 — 0.08 0.10 0.02 8 4.8 2.2 — — 0.10 0.08 — 0.10 0.01 0.08 0.10 0.02 Comparative 9 1.8 1.4 — — — — — — — 0.08 0.10 0.02 alloys 10 6.7 0.3 — — — — — — — 0.08 0.10 0.02 11 4.8 2.2 0.60 — — — — — — 0.08 0.10 0.02 12 6.4 1.4 — 0.16 — — — — — 0.08 0.10 0.02 13 3.6 4.0 0.20 0.16 — — — — — 0.08 0.10 0.02 14 4.8 2.2 0.15 — 0.30 — 0.05 — — 0.08 0.10 0.02 15 4.8 2.2 — 0.10 0.05 0.18 — — — 0.08 0.10 0.02 16 4.8 2.2 0.15 — 0.06 0.07 — — 0.08 0.10 0.02

TABLE 2 Homogenization Hot rolling Cold rolling Alloy First soaking Second soaking Hot rolling Process number Soaking Cooling Cooling stop End-point start annealing in Soaking temperature Soaking rate temperature temperature Holding temperature Temperature Category Number Table 1 method (° C.) time(hr) (° C./hr) (° C.) (° C.) time(hr) (° C.) (° C.) × time Examples 1 1 double 450 3 40 RT 400 4 400 480 × 0.1 s 2 2 double 430 5 30 RT 380 6 380 400 × 6 hr 3 3 double 450 3 40 RT 400 4 400 not performed 4 4 double 450 3 40 RT 400 4 400 480 × 0.1 s 5 5 double 450 3 40 RT 400 4 400 480 × 0.1 s 6 6 double 430 5 40 RT 420 4 420 480 × 0.1 s 7 7 double 450 5 40 RT 430 3 420 480 × 0.1 s 8 8 double 440 5 40 RT 420 4 400 480 × 0.1 s Comparative 9 1 double 450 3 40 RT 400 4 400 480 × 0.1 s Examples 10 1 single 500 3 15 RT — — 450 not performed 11 1 double 510 3 20 RT 400 2 470 530 × 0.1 s 12 1 double 510 3 20 RT 400 2 470 530 × 0.1 s 13 3 double 450 3 40 RT 400 4 400 480 × 0.1 s 14 3 double 450 3 40 RT 400 4 400 480 × 0.1 s 15 3 double 450 3 40 RT 400 4 400 480 × 0.1 s 16 9 double 450 3 40 RT 400 4 400 480 × 0.1 s 17 10 double 450 3 40 RT 400 4 400 480 × 0.1 s 18 11 double 450 3 40 RT 400 4 400 480 × 0.1 s 19 12 single 500 3 15 RT — — 450 not performed 20 13 double 510 3 20 RT 470 4 400 530 × 0.1 s 21 14 double 450 3 40 RT 400 4 400 480 × 0.1 s 22 15 double 450 3 40 RT 400 4 400 480 × 0.1 s 23 16 double 450 3 40 RT 400 4 400 480 × 0.1 s Cold rolling Solution treatment Cold rolling Average rate of reduction temperature rise(° C./s) Process annealing (%) in final Between Holding Average Cooling means cold rolling 300° C. 300° C. 450° C. temperature Holding cooling Category Number or cooling rate step or lower and 450° C. or higher (° C.) time(s) rate(° C./s) Examples 1 fan air cooling 55 0.02 30 0.05 500 60 25 2 30° C./hr 60 1 60 1 500 30 80 3 not performed 90 0.2 30 5 475 30 60 4 fan air cooling 55 5 15 10 515 60 25 5 fan air cooling 70 10 100 6 505 60 25 6 fan air cooling 55 0.5 50 1 500 30 80 7 fan air cooling 60 2 20 0.5 490 30 80 8 fan air cooling 55 2 20 2 485 120 80 Comparative 9 fan air cooling 30 20 20 2 500 60 10 Examples 10 not performed 30 0.02 20 2 500 60 10 11 fan air cooling 55 15 20 2 500 60 10 12 fan air cooling 55 5 8 12 500 45 5 13 fan air cooling 55 0.02 3 0.05 480 60 25 14 fan air cooling 55 0.02 110 0.05 480 60 25 15 fan air cooling 55 0.02 30 18 480 60 25 16 fan air cooling 55 0.1 70 0.1 500 40 25 17 fan air cooling 55 0.1 70 0.1 500 60 25 18 fan air cooling 55 0.1 70 0.1 500 60 25 19 not performed 30 12 20 2 500 60 10 20 fan air cooling 40 5 8 12 500 45 5 21 fan air cooling 55 0.1 70 0.1 500 60 25 22 fan air cooling 55 0.1 70 0.1 500 60 25 23 fan air cooling 55 0.1 70 0.1 500 60 25

TABLE 3 Continued from Table 2 Aluminum alloy sheet Aluminum alloy sheet after solution treatment(T4) after artificial aging(T6) Texture Ratio of surface part Mechanical Mechanical Corrosion to thickness-central part properties properties resistance Surface part Ratio of [surface Ratio of [surface 0.2% 0.2% VDA Grain- Alloy Average [Surface [Surface part Cube] to part S] to Yield Yield bending boundary number grain part part [thickness-central [thickness-central strength strength angle corrosion Category Number Table 1 size(μm) Cube](%) S] (%) part Cube] part S] (MPa) (MPa) (degree) resistance Examples 1 1 36 12 11 1.75 0.57 233 374 58 ∘ 2 2 28 15 10 1.23 0.92 307 425 44 ∘ 3 3 38 31 12 2.31 0.39 214 358 67 ∘ 4 4 18 11 24 1.08 0.95 331 446 40 ∘ 5 5 24 18 14 1.21 0.78 221 351 61 ∘ 6 6 15 11 29 1.05 0.92 346 430 41 ∘ 7 7 34 24 13 1.87 0.68 301 391 59 ∘ 8 8 21 10 12 1.14 0.86 301 386 55 ∘ Comparative 9 1 35 8 14 0.95 1.13 234 375 37 ∘ Examples 10 1 31 11 14 0.82 1.04 232 373 39 ∘ 11 1 34 10 12 0.93 1.09 233 372 38 ∘ 12 1 36 11 11 1.62 1.25 232 375 42 ∘ 13 3 38 12 10 0.98 1.18 211 346 40 ∘ 14 3 35 12 15 1.05 1.32 212 347 41 ∘ 15 3 33 10 16 0.89 1.06 212 346 42 ∘ 16 9 61 37 18 0.97 1.06 107 138 62 ∘ 17 10 53 31 22 1.11 1.27 168 319 53 ∘ 18 11 47 8 15 0.93 1.12 262 331 43 x 19 12 12 1 20 0.92 1.2 218 428 38 ∘ 20 13 11 2 17 0.98 1.0 284 471 32 ∘ 21 14 11 5 22 0.89 1.23 241 369 34 ∘ 22 15 9 2 29 0.85 1.45 248 370 32 ∘ 23 16 10 3 18 0.91 1.29 236 361 35 ∘

While the present invention has been particularly described with reference to specific embodiments thereof, it is obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.

This application claims priority to Japanese Patent Application No. 2015-042565, filed on Mar. 4, 2015, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide 7xxx-series aluminum alloy sheets that are produced via rolling by a common procedure and have better shock absorption (cushing properties) upon automobile collision without lowering their strengths. The present invention is therefore advantageously applicable to structural components typically of automobiles, bicycles, and railroad vehicles, where the resulting structural components contribute to weight reduction. 

1. An aluminum alloy sheet, comprising: in mass percent, Zn in a content of 2.0% to 9.0%; Mg in a content of 0.5% to 4.5%; Cu in a content of 0.5% or less; Zr in a content of 0.15% or less; Mn in a content of 0.2% or less; Cr in a content of 0.15% or less; Sc in a content of 0.05% or less; and wherein the aluminum alloy sheet includes a surface part ranging from a sheet surface to a depth of 15% of a sheet thickness and a thickness-central part, contains grains with Cube orientation and grains with S orientation, an area percentage of the grains with Cube orientation of all grains in the surface part is represented by [surface part Cube], and an area percentage of the grains with S orientation of all grains in the surface part is represented by [surface part S], the thickness-central part contains grains with Cube orientation and grains with S orientation, an area percentage of the grains with Cube orientation of all grains in the thickness-central part is represented by [thickness-central part Cube], and an area percentage of the grains with S orientation of all grains in the thickness-central part is represented by [thickness-central part S], the surface part comprises an equiaxial recrystallized microstructure having an average grain size of 40 μm or less, and the surface part and the thickness-central part have such different textures that: the [surface part Cube] is 10% or more; the [surface part S] is 10% to 40%; a ratio of the [surface part Cube] to the [thickness-central part Cube] is greater than 1.0; and a ratio of the [surface part S] to the [thickness-central part S] is less than 1.0.
 2. The aluminum alloy sheet according to claim 1, further comprising: at least one of (a) and (b), in mass percent: (a) at least one of: Ag in a content of 0.01% to 0.2%, and Sn in a content of 0.001% to 0.1%; and (b) Ti in a content of 0.001% to 0.1%.
 3. A shock absorber, comprising the aluminum alloy sheet according to claim
 1. 4. A shock absorber, comprising the aluminum alloy sheet according to claim
 2. 